●
Initially the Klenow fragment
from the E. coli DNA polymerase I was used in PCR reactions

●
While the Klenow fragment worked
in PCR, it rapidly denatured at the 90°C or higher temperatures required for
denaturation, requiring the addition of new Klenow polymerase after every
denaturation step

●
The need to open the reaction
chamber with every PCR thermocycle made PCR cumbersome and greatly increased
the chances for contamination with unwanted DNA

●
The solution to this problem
came from the discovery and cloning of a thermostable DNA polymerase from the
thermophilic bacterium Thermus aquaticus

●
The polymerase, known as the Taq
polymerase, has many attributes that are well suited for PCR:

●
(a) the enzyme works best at
75-80°C, allowing the elongation step to occur at temperatures which make
non-Watson-Crick base paring a rare event

●
(b) its half-life at 95°C is
over 40 minutes

●
(c) at 72°C it can add 1,000
nucleoside triphosphates to a growing DNA strand

●
Most PCR reactions done with Taq
can be completely closed through all the thermocycles

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